The majority of weather radar systems are generally comprised of multiple components, such as a transmitter, a rotating antenna which includes a reflector, a waveguide, a receiver, multiple RF rotary joints, and associated electronics. In the case of weather radar, electromagnetic energy, or electromagnetic waves, are used to detect, identify, track, and study hydrometeors (i.e. rain, ice crystals, hail, graupel, and snow) and other weather formations. The various components cooperate so that electromagnetic waves can be produced, transmitted, detected and processed.
The transmitter, which generates the desired electromagnetic wave, is typically located on the ground. Most often, the transmitter is located at the base of a tower structure. The tower structure elevates the antenna for the purpose of reducing interference with ground clutter and improving an effective operational range of a system. Antennas often incorporate reflectors for focusing transmitted waves and for amplifying received waves that have reflected from objects. Antennas also incorporate orthomode feed horns for directing and receiving electromagnetic waves from the reflector. Generally, the reflector and the orthomode feed horn rotate to provide a panoramic view of the horizon. Elevating and rotating the reflector creates a number of problems in the prior art.
First, to transport the electromagnetic waves from the transmitter to the reflector, waveguides are installed. Since the waveguides must reach from the transmitter to the reflector, they may be hundreds of feet long. Besides being expensive, long runs of waveguides attenuate electromagnetic waves as they travel from the transmitter to the orthomode feed horn and from the orthomode feed horn to the receiver. Even small losses per foot of waveguide create large cumulative losses over the length of the waveguide. To compensate for these losses, the transmitters must have peak powers that exceed the system's targeted transmission power. Therefore, in addition to the capital cost incurred to install the waveguide, excess capital is spent to oversize the transmitter.
Waveguides are also problematic from an operational expense viewpoint. Since the waveguide extends from the orthomode feed horn to the ground based transmitter, a portion of the waveguide may be exposed to moisture in the environment. As with many other types of electronics, waveguides are sensitive to moisture. Minute quantities of moisture may have deleterious effects on the electromagnetic waves as they pass through the waveguide. Various waveguide installation designs attempt to minimize the effects of water on waveguide operation. For instance, some designs use a purge gas, such as dry air or nitrogen, to pressurize the waveguide, thus inhibiting penetration of moisture into the waveguide. Continuous flow of the purge gas is usually required since small gas leaks develop over time. Thus, in addition to being expensive to purchase, waveguides are expensive to operate.
Second, since the antenna rotates, and the transmitter and waveguides do not, connectivity between the waveguide and the rotating antenna is critical to system performance. RF rotary joints are commonly used to transfer the electromagnetic energy between the stationary guide and the rotating reflector. To complicate the connectivity problems, the reflector may have azimuth and elevation movement. In other words, the antenna moves about two axes. Therefore, two RF rotary joints per waveguide must be used, or alternatively a special RF rotary joint having two axes of movement may be installed. In most cases, the drawbacks to RF rotary joints include (a) significant power loss and phase distortion as the electromagnetic wave transitions through the RF rotary joint, (b) they are likely points of water intrusion, and (c) they are high wear components. In summary, like waveguides, RF rotary joints are expensive to install and reduce the performance of the radar system.
Therefore, what is missing in the art is a radar system lacking RF rotary joints and long runs of waveguide between the transmitter and the orthomode feed horn. Furthermore, what is missing is a dual-polarization simultaneous-emission weather radar system having low capital and operating cost with superb performance.